During laparoscopic surgery, one or more small incisions are formed in the skin of a patient and a trocar assembly is inserted through the incision to form a pathway that provides access to an internal body cavity, such as the patient's abdomen. The trocar assembly is used to introduce various instruments and tools into the abdomen, as well as to provide insufflation that elevates interior walls of the abdomen.
A trocar assembly generally includes a cannula, an obturator, and a housing. To set the trocar assembly for a surgical operation, the obturator is extended through an inner lumen of the cannula and is used to pierce through the patient's skin to access the abdominal cavity. To penetrate the skin, the distal end of the cannula is placed against an incision in the skin and pressure is applied to the proximal end of the trocar to force the sharp point of the obturator through the skin until it enters the targeted body cavity. The obturator can then be withdrawn, leaving the lumen of the cannula as a path to access the abdominal cavity from outside the body.
The trocar housing is attached to the proximal end of the cannula and defines a working chamber with an open distal end in communication with the lumen of the cannula. Just as the inner can receive the obturator, it is also sized to receive elongated surgical tools that are axially extended into and withdrawn from the cannula through the proximal end portion of the working chamber.
For surgical operations, a surgeon will normally use a 1:1 pairing of a trocar assembly and a surgical tool. For example, if an 8 mm (diameter) surgical tool is required for an operation, a corresponding 8 mm (diameter) trocar assembly will be used. In robotic surgery, however, trocar assemblies and surgical tools will not always enjoy a 1:1 pairing. For example, 12 mm (diameter) trocar assemblies are typically used in robotic surgery, which enables use of 12 mm (diameter) surgical tools, such as a surgical stapler. Yet some procedures require an 8 mm or 5 mm (diameter) surgical tool, which will have to pass through the 12 mm trocar assembly.
When the trocar assembly and surgical tool pairing is not 1:1, there can be “lost motion” or hysteresis to the movement where the tip (distal end) of the surgical tool is prone to various types of unintended motion, such as deflection, oscillation in place, and spring back oscillation. For instance, the initial movement input to the robot by the surgeon will not move the surgical tool, but will instead first remove the clearance between the instrument and the trocar assembly, and will subsequently move the surgical instrument. This lost motion is unacceptable to the surgeon, who expects fine control and precision from the surgical robot. The clearance between the trocar assembly and the smaller diameter surgical instrument can also result in the surgical instrument vibrating without hitting the inner walls of the trocar assembly.